GSHPs have been used on a limited basis since the late 1940s. Closed-loop GCHPs were investigated in several projects (Ingersoll et al. 1954; Kemler and Oglesby 1950; Penrod 1954; Vestal and Fluker 1956). The concept was not commercialized because of low energy costs and material limitations. GSHPs became popular in the Gulf Coast region in the 1960s because of the abundant supplies of water. However, the current interest in GSHPs, especially GCHPs, began soon after the energy crisis of the mid-1970s. The initial emphasis was an attempt to couple the concept with solar collectors (Bose 1984). A later study concluded that the concept was effective without the solar assist (Parker et al. 1984). At this time, the fledgling industry adopted the use of thermally fused plastic pipe as a method of reducing cost and improving reliability.

Commercialization began in rural climates in which heating requirements were dominant and single-pipe horizontal systems were widely used. As the market slowly expanded into semi-rural areas with smaller lots, vertical U-bend piping loops and horizontal multiple pipes and coils were used. The water-to-air heat pumps used in these installations were originally intended for water-loop heat pump (WLHP) applications. Some modifications were made to accommodate lower entering liquid temperatures. However, advanced water-to-air heat pumps were not developed because of the small market share of GCHPs. In the late 1980s, advances were made to accommodate the growing market. Improvements in efficiency were made by incorporating larger heat exchangers, more efficient fan...